Conventionally, concrete composition contain a cementitious material, for instance cement based on Ordinary Portland Cement, mineral additions, inorganic additions, water, sand aggregate, fine and coarse aggregates as well a various admixtures to reduce the water demand (water reducers or dispersants), superplasticizers, viscosity modifiers, accelerators, air entrainers, and additives like fillers, fibers, etc.
Impurities contained in the mineral, in the sand and aggregates (for instance various traces of clay like smectite, montmorillonite, koalinite, illite, etc.) may not only strongly affect the availability of water in the fresh concrete but will also influence the efficiency of the admixtures that will be quickly adsorbed by the impurities, thus limiting their core effect in the concrete. As a result, concrete mix designs are highly unstable and lack robustness so fresh and final properties are dramatically impacted. Furthermore, overdosing the admixtures to compensate for the adsorbed part will produce unexpected degradation of the mix designs in term of stability (segregation between cement paste and aggregates) or mechanical properties (intermediate and final strength).
Such a mechanism is also observed using aggregates from various geological sources. Rocks like schist, micaschist gneiss, shale, slate may phyllosilicates like kaolinite, talc, micas, serpentines, chlorite, smectites, etc.
Moreover, phyllosilicates are known to degrade into clay minerals (illite-smectite clays from muscovite and paragonite mica) and in certain conditions can release the interlayer cations leading to increased interactions.
The main properties that influence the reactivity are typically                Particle dimensions: specific surface, shape        Superficial charge (quantity of charge in the surface)        Cationic exchange capacity        Structure interlayer: type of cation, dimension of interlayer        
EP 1 799 624 B1 describes the use of cationic polymers with high level of charge to inert the negative effect of clay traces in sand used in concrete formulation. The use of such polymers in real concrete mix designs is, however, not described and limited and no information are available on the interaction with concrete technology admixtures like superplascizers, retardants or workability extension admixtures. Data are available for workability retention on mortar tests. Tests are limited to clay in sand, not exceeding 1% (m/m) of the sand and the presence of layered material in the aggregates is not mentioned.
EP 1 015 398 describes the combination of a cationic polymer together with a comb-type polymer (plasticizer), tested on mortar samples. A cationic polymer as in EP 1 799 624 B1 is used to limit or inhibit the effect of impurities on the depletion of water and plastizing admixture, thus obtaining acceptable workability with acceptable comb-polymer dosages. No results are provided concerning the workability retention (e.g. the capacity to maintain the rheological, or the slump/spread properties) obtained after mixing over a significant period of time (40 minutes to some hours), this enabling transportation with no degradation of the properties of the concrete to be placed.
The use of cationic polymers in concrete presents some serious drawbacks. The most important problem is connected with the fact that these cationic polymers contain chloride that is not recommended in concrete applications due to the corrosion of the reinforcement rebars. Also, these substances are not widely produced industrially and the supply may be problematic.
Finally, the technologies described in the prior art cannot always be used directly in the concrete mix designs as a normal admixture, but require a pre-processing of the contaminated sand aggregates or aggregates prior to their utilization in concrete mixes.